(a) Field of the Invention
The invention is directed to a process for imaging image points of a video picture by means of a light bundle which is controlled in intensity with respect to the brightness of the image points and which is deflected and then projected on a screen by a lens or a lens system. Further, the invention is directed to an apparatus, in particular for carrying out the process and especially for the imaging of image points of a video picture on a screen, with a deflecting device which deflects a light bundle onto a screen for linewise and framewise raster-scanning, this light bundle being controlled in intensity with respect to the brightness of the image points, and with a lens or a lens system between the deflecting device and the screen.
(b) Description of the Related Art
In conventional video technique using picture tubes, the image size is very limited due to high cost. However, other techniques known from the literature can be used to generate very large color video pictures. For example, devices are described in an article in Funkschau 1970, 4, pages 69ff. and in EP-OS 0 084 434 in which three light bundles of different colors generated by lasers are intensity-modulated by light modulators and combined via a mirror system to form a common light bundle which is mechanically raster scanned by means of a mirror system and is projected onto a screen.
In professional applications, large-image projectors are also currently being used. However, they are based on a different imaging principle. Regardless of the technical principle, however, large-image projectors are subject to the same requirements regarding image size. Due to their high price, these projectors are seldom permanently installed, but rather are rented from rental firms for special cases. Therefore, such large-image projectors must be kept flexible for many kinds of applications. In practice, such devices can be used in small cinemas as well as in football stadiums. For this reason, there is a considerable need for variable image size.
There has been no satisfactory solution to date for imaging a video picture with laser light bundles as described above. A magnification or reduction of the image size can be achieved, for instance, by mechanically modifying or exchanging the deflecting device, but this is complicated and expensive. Above all, this possibility is not practicable for large projection devices which are hired out to various facilities, since operators usually having scant technical ability cannot be expected to constantly readjust the mirrors for the purpose of the respective case of application at the place of use.
For the magnification of the video picture in video devices operating with laser technology, DE-OS 43 24 849 suggests the use of a substantially afocal lens system by which the image field can be expanded. This lens system enables a flatter type of construction of such video systems, for example. However, magnification is fixed in the lens system described in this reference, since it is designed primarily to achieve magnification with minimum chromatic errors and distortion. Since the lens system is optimized for reducing imaging errors, it can hardly be assumed that this optimum can be maintained, e.g., by varying the imaging characteristics of a partial system. Therefore, another solution must be sought.
In order to change the angle of an exiting light beam relative to the incident light beam, f(.theta.) lenses in which the angle is adjusted according to a predetermined function f are also known. For instance, lenses which use a tangent function for angular adjustment in order to transform a uniform angular deflection for uniform raster scanning in a plane are used in technical applications.
However, such optical systems do not enable magnification. Above all, an analysis contained in DE-OS 43 24 849 indicates that such f(.theta.) lenses cannot be constructed so as to be free of distortion and free of chromatic errors at the same time within acceptable tolerances, which indicates that there is no suggestion to change the image size in a variable manner in video technology in connection with f(.theta.) lenses.
Variable-focus objectives for use in cameras or film projectors are known from the prior art. However, they are only suitable for imaging a flat picture on a screen, as in the magnification of a slide on a projection screen. However, in video systems or laser printers, an angle which is generated by the deflection of the mirrors of the deflecting device must be converted into a deflection on a plane screen. Thus, the image would first have to be transformed into an image with a f(.theta.) lens before projecting it by means of a known variable-focus objective. As is stated in DE-OS 43 24 849, this cannot be carried out without distortion and without chromatic errors at the same time.
In summary, it must therefore be asserted that there has been no satisfactory solution to date for a variable changing of the image size in laser video systems. However, variable adjustment of image size is necessary particularly for the flexible use of video devices in professional applications for different spatial conditions.